Eur. J. Entomol. 105: 513–520, 2008 http://www.eje.cz/scripts/viewabstract.php?abstract=1359 ISSN 1210-5759 (print), 1802-8829 (online)

Corn leaf , maidis (: ), is a key to greenbug, graminum (Hemiptera: Aphididae), biological control in grain ,

GERALD J. MICHELS, JR. and JAMES H. MATIS

Texas Agricultural Experiment Station, P. O Drawer 10, 2301 Experiment Station Road, Bushland, TX 79012, USA; e-mail: [email protected] Department of Statistics, Texas A&M University, College Station, TX 77843-3143, USA; e-mail: [email protected]

Key words. Corn leaf aphid, greenbug, coccinellids, biological control, grain sorghum, management

Abstract. In the sorghum/aphid/ladybeetle ecosystem found in the Texas High Plains Region of the United States, we found that the corn leaf aphid, (Fitch), is a key aphid species that provides a critical early-season food source for native coc- cinellids. From 1988 to 2000 data on the seasonal abundance of sorghum-infesting and ladybeetles were collected from a total of 21 irrigated and 12 rain-fed grain sorghum fields. The data indicated that successful biological control of the greenbug by cocci- nellids is normally dependent on early-season colonization of the sorghum field by corn leaf aphids. When corn leaf aphids exceeded 100/plant before sorghum boot stage greenbugs never exceeded 125 aphids/plant. In all cases where greenbugs were found in densi- ties that would cause economic damage to sorghum (>250/plant), corn leaf aphids reached a density of 100 or more per plant after sorghum reached the boot stage. In irrigated fields, the first record of coccinellid eggs and peak coccinellid abundance were posi- tively and significantly (p = 0.05) correlated to the day of the year when corn leaf aphids reached or exceeded a minimum of 100/plant and corn leaf aphid peak abundance in both irrigated and rainfed fields. On the other hand, greenbug peak abundance was significantly correlated only to coccinellid peak abundance in irrigated fields. Regression analyses indicated that in paired analyses of irrigated and rain-fed sorghum fields, an increase of one aphid at time t, resulted in an increase in coccinellid peak abundance from 0.024 to 0.025 per 15 m of row at time t + 2 depending on aphid species, if corn leaf aphids reached a level of 100 or more per plant by sorghum boot stage and irrigation parameters. We concluded that corn leaf aphids are an important early-season food source for predaceous coccinellids, drawing these predators into the fields where they feed on the aphids and deposit eggs, engendering a captive larval population that is present when greenbug first begin to enter the field later in the season.

INTRODUCTION rarely cause economic loss and can be considered helpful At times, pest management practices can concentrate by attracting coccinellids. Cate et al. (1973) and Wilde & too narrowly on a specific predator prey interaction Ohiagu (1976) both concluded that using insecticides to without bringing into account other factors. The control corn leaf aphids did not increase sorghum yield greenbug, (Rondani), is a serious nor did corn leaf aphid infestations caused significant economic pest of grain sorghum, Sorghum bicolor (L.) damage to grain sorghum. In addition, applying insecti- Moench, in the United States Great Plains. Existing bio- cides for corn leaf aphids disrupts or eliminates predators control agents, primarily coccinellids in the genus Hippo- and parasitoids which attack greenbugs in sorghum or damia, can keep this pest at sub-economic levels if there migrate and aid in biological control of aphids in is a proper understanding of the ecological interactions such as cotton (Cronholm et al., 1998). among the native predaceous coccinellids and their aphid Research by Kring et al. (1985), Kring & Gilstrap prey. (1986), and & Wilde (1988) clearly indicated a rela- In the early 1980s in the High Plains of Texas, aphids tionship between corn leaf aphids and predaceous cocci- infesting grain sorghum, Sorghum bicolor (L.) Moench, nellids which was a key to greenbug, Schizaphis regardless of species, were controlled by aerially-applied graminum (Rondani), biological control. This research insecticides. Timing for the applications was based on prompted us to examine the interactions among the aphids seeing “slick leaves” caused by the deposition of corn leaf and predators over an extended period of time and by irri- aphid, Rhopalosiphum maidis (Fitch), honeydew. This gated and rain-fed agronomic practices, to determine if was an unsophisticated control strategy which typically biological control of greenbugs in grain sorghum could be required multiple insecticide applications for aphids and better understood and enhanced by conserving natural oftentimes resulted in late-season infestations by Banks enemies. This paper presents the results of a multiple-year grass mites, Oligonychus pratensis Banks, for which there study of the seasonal abundance of sorghum-feeding were no effective control measures (Buschman & DePew, aphids and the associated predacious coccinellids. 1990). Cronholm et al. (1998) note that corn leaf aphids

513 MATERIAL AND METHODS 1. To obtain accurate aphid counts, especially corn leaf aphids which are often found deep in the whorl, destructive sampling Field data collection was necessary, From 1988 to 2000, with the exception of 1992, 33 “field- 2. Time and personnel considerations, years” of data on aphids, predators and parasitoids were col- 3. Removing 600 plants per week from a particular field for lected in irrigated and rain-fed grain sorghum fields. A list of destructive sampling during the season could have adverse the aphid, predator and parasitoid species collected during the effects on the distribution of aphid and beneficial . research is found in Table 1. A “field year” is defined as one The same sampling routine was carried out each year. We field sampled throughout the growing season. In Bushland, began observing fields in early June and sampling started when Potter Co., TX, data were collected for 12 years in irrigated and corn leaf aphids first began to appear in the field and continued four years in rain-fed grain sorghum fields. In Etter, Moore Co., on a weekly basis until greenbug densities dropped to zero for TX, data were collected for seven years in irrigated and six two consecutive weeks. At times, heavy rains prevented sam- years in rain-fed grain sorghum fields. Two additional fields pling on a strict seven-day interval. However, samples were were sampled in Oldham Co. (irrigated) and in Gray Co., TX, taken as closely as possible to the planned sampling date (Table (rain-fed) in 1994 and two fields were sampled in Parmer Co., 2). TX, (one irrigated and one rain-fed) in 1995. Standard agro- Analyses of parasitoids in relation to initiating greenbug bio- nomic practices for grain sorghum production were followed logical control were omitted from this paper. Although parasi- each year. toids are important to the overall seasonal abundance of green- Fifty consecutive plants down a field row (approx. 15 m) bugs, and to a lesser extent, corn leaf aphids, in this region, from a random starting point were examined by hand. Predator parasitoids occur late in the season and perform more of a and parasitoid numbers/plant were recorded on standardized “clean-up” function rather than being critical to suppression of data sheets. After completing predator and parasitoid sampling, greenbugs when they can most damage gain sorghum. 12 plants were randomly selected from within the 50-plant sam- ple, cut off at the base, and all corn leaf aphids and greenbugs Data analyses were counted. The process was repeated 12 times at each sam- Pearson’s Correlation Coefficients comparing corn leaf aphid pling date, giving a total of 600 plants sampled for predators and peak abundance, the date when corn leaf aphids averaged parasitoids and 144 plants sampled for aphids. 100/plant, and greenbug peak abundance to the occurrence of Different numbers of plants were sampled for natural enemies the first predacious coccinellid eggs and the peak coccinellid and aphids for three reasons: predator abundance were determined using SAS ver. 8.0 (1999) PROC CORR. Regression analyses were performed using SPSS

TABLE 1. Aphid, predator and parasitoid species collected in grain sorghum fields in the Texas High Plains from 1988–2000. Family Percentage Percentage of Total recorded Genus and species of all samples 1 Coccinellidae all predators parasitoids aphids Aphididae Rhopalosiphum maidis (Fitch) 297,363 43.21 53.19 Schizaphis graminum (Rondani) 261,721 57.15 46.81 Coccinellidae (adults) Hippodamia convergens Guérin-Ménéville 18,652 56.96 61.46 57.05 Hippodamia sinuata Mulsant 9,962 36.58 32.82 30.47 Other 996 12.71 3.28 3.05 Scymnus (Pullus) loweii 342 5.26 1.13 1.05 Coleomegilla maculata Timberlake 302 4.05 1.00 0.92 Coccinella septempunctata L. 60 2.37 0.20 0.18 Hippodamia parenthesis (Say) 24 1.75 0.08 0.07 Olla v-nigrum (Mulsant) 12 0.26 0.04 0.04 Coccinellidae egg masses 1,689 Coccinellidae larvae 14,842 Coccinellidae pupae 4,170 Chrysopidae Chrysoperla sp. larvae 1,103 40.15 3.37 Chrysoperla sp. eggs 9,775 Nabidae Nabis americoferus Carayon 1,205 8.57 3.68 Syrphidae (larvae) 39 4.94 0.12 Aphelinidae Aphelinus varipes (Foerster) mummies 19,590 11.03 19.48 Braconidae (Cresson) mummies 80,972 19.34 80.52 1 Percentage of all samples containing the specified , 4224 samples total.

514 TABLE 2. Day of the year for first and last occurrence of aphids and predacious coccinellids in grain sorghum at five sites in Texas, 1988–2000. RM 1 SG HC HS RM SG HC HS Sorg- RM SG HC HS RM SG HC HS Sorg- hum hum Year Potter County irrigated Potter County rainfed boot Moore County irrigated Moore County rainfed boot stage stage Season intiation (day of year) Season intiation (day of year) 1988 174 174 174 174 199 1989 166 166 157 171 199 1990 156 156 156 163 187 1992 181 212 181 181 203 1993 182 199 182 182 194 1994 173 194 173 173 188 177 198 177 177 189 1995 192 213 176 176 202 193 207 207 212 180 180 200 207 201 1996 171 210 171 171 189 178 178 178 178 178 178 178 178 191 1997 180 208 187 180 194 208 194 194 197 176 182 176 176 176 176 176 176 194 1998 174 188 174 169 195 202 180 180 186 201 201 194 187 187 194 187 216 187 1999 185 185 201 222 185 185 194 194 197 166 166 166 180 166 166 166 173 193 2000 166 166 186 186 166 166 186 208 194 170 170 181 187 181 170 181 181 Avg. 175.1 190.6 176.7 179.5 185.0 190.3 188.5 194.0 194.2 180.1 186.0 182.7 185.3 178.0 177.3 181.3 188.5 192.5 SE 3.1 6.2 3.9 4.7 6.7 9.4 3.4 5.7 1.8 4.7 6.1 5.1 4.8 2.8 4.0 4.7 7.4 2.0 Season termination (day of year) Season termination (day of year) 1988 265 265 265 265 1989 241 241 241 241 1990 240 240 255 255 1992 246 253 253 253 1993 245 245 238 245 1994 235 251 251 251 227 233 240 240 1995 250 250 250 250 243 264 256 250 264 264 256 256 1996 234 241 241 234 220 226 226 226 226 226 226 226 1997 258 258 258 258 258 258 258 258 253 253 253 253 253 246 253 246 1998 230 244 244 237 237 237 244 237 216 236 243 236 236 236 236 243 1999 228 235 222 222 235 235 228 228 236 236 236 230 220 236 230 173 2000 222 240 228 222 212 232 208 208 221 221 221 193 227 213 235 209 Avg. 239.0 245.3 243.7 242.5 235.5 240.5 234.5 232.8 230.9 238.4 239.3 232.6 237.7 236.8 239.3 225.5 SE 3.2 2.1 3.4 3.8 9.4 5.9 10.8 10.4 5.2 5.7 4.9 7.6 7.1 7.1 5.0 12.5 Season initiation (day of year) Oldham County irrigated Gray County rainfed 1994 175 209 168 168 193 207 173 186 — 2 Parmer County irrigated Parmer County rainfed 1995 191 219 174 177 184 212 212 184 — Season termination (day of year) Oldham County irrigated Gray County rainfed 1994 256 256 256 256 250 250 250 250 Parmer County irrigated Parmer County rainfed 1995 263 263 263 263 263 263 263 263 1 RM – Rhopalosiphum maidis, SG – Schizaphis graminum, HC – Hippodamia convergens, HS – Hippodamia sinuata. 2 Boot stage not recorded. ver. 13.0 (2007). The procedure was to regress peak coccinellid and t the predictions hold little practical value. This makes abundance at sampling time t on aphid abundance at sampling sense, in that predator abundance tends to follow prey abun- time t – 2. This “look back” approach was employed using dance, and there should be some optimal “look back” time that various steps back, with t – 2 being found to be the most rele- provides the best fit of the data. vant. Prior to t – 2, the predictions were too weak, and at t – 1

515 Fig. 1. Greenbug, corn leaf aphid, predacious coccinellid and Fig. 2. Greenbug, corn leaf aphid, predacious coccinellid and parasitoid abundance in irrigated grain sorghum fields, Bush- parasitoid abundance in rain-fed grain sorghum fields, land, TX, 1988–2000. Sorghum boot stage represented by ver- Bushland, TX, 1988–2000. Sorghum boot stage represented by tical lines. vertical lines.

RESULTS AND DISCUSSION “trigger” in biological control of greenbugs in grain sor- ghum because the appearance of corn leaf aphids early in Figs 1–6 illustrate the seasonal occurrence and abun- the season probably initiates coccinellid oviposition, thus dance of corn leaf aphids, greenbugs, predacious cocci- ensuring a “captive” population of larvae that are present nellids and parasitoids in irrigated and rain-fed grain sor- to feed on greenbugs as they colonize the field. Failure of ghum fields located in Bushland, Potter County, TX (Figs corn leaf aphids to colonize a field, or approach an abun- 1, 2); Etter, Moore County, TX (Figs 3, 4); and Gray, dance of 100/plant by the boot stage of sorghum probably Oldham, and Parmer Counties, TX (Figs 5, 6). results in few coccinellids ovipositing, and greenbugs not In irrigated grain sorghum fields (Table 3), corn leaf being subjected to predation sufficient to keep their num- aphid and greenbug peak abundance were significantly bers below economically damaging levels. correlated to peak coccinellid abundance (p = 0.005 and For greenbug management in either irrigated or rain-fed 0.008, respectively). Corn leaf aphid peak abundance and grain sorghum, a seemingly handy “rule of thumb” is that the date when corn leaf aphid density averaged 100/plant greenbug biological control by coccinellids is successful were also significantly correlated to first coccinellid eggs and outbreaks do not occur when corn leaf aphids reach in irrigated fields (p = 0.007 in both instances). Greenbug or exceed 100/plant by the time sorghum reaches the boot peak abundance was not significantly correlated to the stage. appearance of the first coccinellid eggs. Regression analyses (Table 4) indicated that peak coc- In rain-fed sorghum fields, the same results were found cinellid abundance may be predicted by greenbug abun- with the exception that greenbug peak abundance was not dance or a combination of greenbug and corn leaf significantly correlated to coccinellid peak abundance. abundance, looking back to the aphid abundance two The results from the correlation analyses coupled with the weeks prior to the coccinellid peak. When all fields were seasonal abundance of corn leaf aphids and greenbugs considered, ignoring whether they were irrigated or rain- presented in Figs 1–6 may indicate a critical factor or fed, the regressions indicated that when one greenbug is

516 Fig. 3. Greenbug, corn leaf aphid, predacious coccinellid and Fig. 4. Greenbug, corn leaf aphid, predacious coccinellid and parasitoid abundance in irrigated grain sorghum fields, Etter, parasitoid abundance in rain-fed grain sorghum fields, Etter, TX, 1995–2000. Sorghum boot stage represented by vertical TX, 1995–2000. Sorghum boot stage represented by vertical lines. lines. added to the system at time t, the coccinellid peak abun- side at the same location. In this analysis, if greenbugs dance increases by 0.033 at time t + 2. If corn leaf aphids alone were used as the independent variable, one addi- and greenbugs were combined as a single number, tional greenbug at time t increased peak coccinellid abun- included in the equation as separate parameters, or if an dance per 15 m of row at time t + 2 by 0.024 (R2 = 0.77, p indicator variable (I) was added to different fields where = 0.024). If aphid species are combined, then coccinellid corn leaf aphids averaged 100/plant by boot stage (I = 1 peak abundance per 15 m of row increased 0.025 at t + 2 for corn leaf aphids averaging 100/plant or more by boot for each aphid added a t (R2 = 0.77, p = 0.025). If an indi- stage, 0 if not), the results were similar; the increase in cator variable (I) was added as a parameter to the pre- peak coccinellid abundance ranging from 0.034 to 0.037. vious equation to distinguish irrigated from rain-fed fields A more practical way of looking at these results would be (I = 1 for irrigated fields and 0 for rain-fed fields), the to express the increase in peak coccinellid abundance in same result is found with a slightly higher R2 but a much terms of an increase of 1000 aphids; e. g. for Equation 1 lower p (0.78 and 0.000, respectively). in Table 4, an addition of 1000 greenbugs at time t Looking at a graph of the regression analyses (Fig. 7) in increases the peak coccinellid abundance by approxi- rain-fed fields vs. irrigated fields, a smaller complement mately 35 at t + 2. Although these regressions were sig- of initial coccinellids was indicated in rain-fed fields. nificant, with p ranging from 0.009 to 0.035, the This is plausible and could be due to slower plant devel- 2 coefficient of determination (R ) was very low in all opment and corn leaf aphid colonization in rain-fed fields, cases, ranging from 0.19 to 0.21. resulting in lower coccinellid oviposition. Although pre- Better results were found when the analyses were run cipitation varies on a yearly basis, irrigated fields with pairs of irrigated and rain-fed fields from the same invariably receive more moisture than rain-fed fields in location. Therefore, instead of lumping 33 “field-years” the semi-arid environment of the Southern Great Plains. of information together, we examined 10 pairs of fields A graph of the regressions for greenbugs alone vs. a where irrigated and rain-fed fields were sampled side-by- combination of both aphid species indicated that if corn

517 Fig. 5. Greenbug, corn leaf aphid, predacious coccinellid and Fig. 6. Greenbug, corn leaf aphid, predacious coccinellid and parasitoid in irrigated grain sorghum fields, Gray, Oldham, and parasitoid in rain-fed grain sorghum fields, Gray, Oldham, and Parmer, Co’s, TX, 1994, 1995. Sorghum boot stage represented Parmer, Co’s, TX, 1994, 1995. Sorghum boot stage represented by vertical lines. by vertical lines. leaf aphids and greenbugs were considered together as a The results of both the correlation and regression common food source, a better indication of the initial coc- analyses indicate that it is probably better for research cinellid complement (y-intercept) was achieved than projects to experimentally pair irrigated and rain-fed when greenbugs were considered alone. The greenbug- fields in the same location within the same year. Irrigation only regression inferred a higher coccinellid abundance is a significant factor in the number of aphids observed in being present when the greenbugs first entered the field, a field (compare Figs 1 to 2, 3 to 4 and 5 to 6), and the which could be due to the presence of corn leaf aphids that are unaccounted for in this equation (Fig. 8). The combination of the correlation and regression TABLE 3. Pearson correlation analyses for the relationship analyses support the concept that corn leaf aphids are between the appearance of the first coccinellid eggs and cocci- indeed instrumental to greenbug biological control in nellid peak abundance to peak corn leaf aphid and greenbug grain sorghum. Although the correlation analyses give abundance and corn leaf aphids averaging 100/plant in irrigated and rain-fed grain sorghum fields, Texas High Plains, stronger support that the simple regression analyses, the 1998–2000. regressions indirectly support the concept. The y-intercepts illustrated in Fig. 8 clearly indicate that if Irrigated Rain-fed corn leaf aphids are not accounted for, coccinellid abun- First Egg Pred Peak First Egg Pred Peak dance in the field is higher when greenbugs first enter the R. maidis peak 0.705 0.663 0.677 0.693 field. Looking at Table 2 and Figs 1–6, it is obvious that Prob. 0.007 0.005 0.010 0.008 in most years corn leaf aphids are present in the field ear- R. maidis = 100 0.705 0.677 0.909 0.865 lier than greenbugs, or if corn leaf aphids and greenbugs are initially found at the same time, corn leaf aphid abun- Prob. 0.007 0.004 0.005 0.012 dance is higher than greenbug. S. graminum peak 0.316 0.638 0.167 0.470 Prob. 0.293 0.008 0.721 0.287

518 Fig. 7. Comparison of linear regression analyses for pre- dicting coccinellid abundance per 15 m of row for irrigated and Fig. 8. Comparison of linear regression analyses for pre- rain-fed grain sorghum fields at time t + 2 using aphids/plant at t dicting coccinellids per 15 m of row in grain sorghum fields at as the independent variable. For irrigated fields I = 0, for time t + 2 using greenbugs alone/plant or any aphid rain-fed fields I = 1. species/plant at t as the independent variable. number of coccinellids that can be expected to be present ACKNOWLEDGMENTS. In a study encompassing thirteen (Fig. 7). years of field data collecting, a great number of people are The results we observed in this research extend the involved. The many summer spent by field crews with the results reported by Kring & Gilstrap (1986) who found Texas Agricultural Experiment Station Entomology Program at that corn leaf aphids maintained H. convergens in sor- Bushland, TX are truly appreciated. We are indebted to R.W. Behle, R.L. Renn, and D.A. Owings who as Research Associ- ghum fields. Rice & Wilde (1988) had similar findings in ates supervised the many field crews from 1988–1990, regard to predators in sorghum. Even with 33 “field 1991–1994, and 1995–2000, respectively. We also acknowledge years” of data, and 10 years of side-by side comparisons J.B. Bible, Research Assistant, who participated in each sum- of irrigated and rain-fed grain sorghum fields, there are mer’s field sampling from 1988–2000. Finally, we thank F.E. still questions that need to be answered. We are comfort- Gilstrap and T.J. Kring who provided insightful comments on able concluding that the general “rule of thumb” men- the research throughout its run. tioned above (that greenbug biological control by coccinellids is successful and outbreaks do not occur REFERENCES when corn leaf aphids reach or exceed 100/plant by the CATE J.H. JR., BOTTRELL D.G. & TEETES G.L. 1973: Management time sorghum reaches the boot stage), and believe it can of the greenbug on grain sorghum. 1. Testing foliar treatments be used with confidence as a management tool. However, of insecticides against greenbugs and corn leaf aphids. J. the results additionally point to a need to thoroughly Econ. Entomol. 66: 945–951. assess climatic conditions, and perhaps look at the role CRONHOLM G., KNUTSON A., PARKER R., TEETES G. & PENDLETON B. 1998: Managing insect and mite pests of Texas sorghum. non-coccinellid predators might play in the system. Tex. Agric. Exp. Stat. Bull. B-1220. 26 pp. Although we did not add such predators as lacewings and BUSCHMAN L.L. & DEPEW L.J. 1990: Outbreaks of Banks grass syrphid fly larvae to our analyses, there is a chance that mite (Acari: Tetranychidae) in grain sorghum following these minor predators play a role in specific years. insecticide applications. J. Econ. Entomol. 83: 1570–1574.

TABLE 4. Linear regressions of aphid variables on coccinellid peak abundance. Equation1 R2 p Result of adding one indicated aphid(s) at time t on C at time t + 2 All fields considered, irrigation ignored. 1. C = 29.5 + 0.033SG 0.19 0.010 C increases by 0.033 2. C = 20.1 + 0.034A 0.20 0.009 C increases by 0.034 3. C = 20.4 + 0.034SG + 0.037RM 0.20 0.035 C increases by 0.034 or 0.037, for SG or RM, respectively 4. C = 11.3 + 24I + 0.037A 0.21 0.027 C increases by 0.037. RM > 100/plant by boot stage increases C by 24 Paired field analyses (1 irrigated, 1 rain-fed field), 10 comparisons, 20 fields. 5. C = 10.8 + 0.024SG 0.77 0.024 C increases by 0.024 6. C = 4 + 0.025A 0.77 0.025 C increases by 0.025, irrigation ignored 7. C = 6–(5*I) + 0.025A 0.78 0.000 C increases by 0.025, irrigation considered. Irrigation increases C by 5 1 A = any aphid species, C = coccinellid peak abundance, RM = R. maidis (corn leaf aphid), SG = S. graminum (greenbug). For Eq. 4, if RM > 100 by boot stage I = 1 otherwise I = 0. For Eq. 7, if irrigated I = 1 otherwise I = 0.

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Received October 16, 2007; revised and accepted May 6, 2008

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